The present invention relates to a new type of optical amplifier apparatus especially, but not exclusively, optical amplifier apparatus intended for use in an optical fiber networks.
Optical fiber is penetrating rapidly in subscriber access networks for distribution of CATV (cable television) services. Today""s CATV headend stations feed distributive services to large numbers of subscribers ( greater than 1000) in networks with abundant splitting in the fiber part as well as in the coaxial part. The present demand for interactive services, like teleworking and high quality videotelephony, is increasing but it requires a high-bandwidth and bi-directional link between servers and customers. To implement such interactive services in CATV networks, it is necessary to develop high definition wave division multiplexing (HDWDM) techniques and devices. One of these essential devices is a bi-directional multiwavelength optical amplifier which has to compensate for the propagation and the splitting losses along the link for both upstream and downstream signals. No such device is currently available.
The present invention provides a new optical amplifier architecture having two parallel optical branches, one for amplifying digital signals and the other for amplifying analogue signals. These optical branches are parallel in the sense that they are connected in parallel such that certain signals pass through one of the branches while others pass, possibly simultaneously, through the other. Preferred embodiments of this optical amplifier apparatus enable simultaneous amplification of an analogue CATV signal and a plurality of multiplexed bi-directional digital signals, compensating the propagation and splitting losses along the link for both upstream and downstream signals.
It is particularly preferred that the optical amplifier apparatus of the present invention should make use of erbium doped fibber amplifier (EDFA) elements. Typically, the wavelengths handled by the digital path in the optical amplifier apparatus will be in the shorter wavelength region (e.g. 1530-1545 nm) of the EDFA gain spectrum whereas the wavelengths handled by the analogue path of the optical amplifier will be in the longer wavelength region (1550-1560 nm). The frequency modulation of the analogue signal often results in a frequency variation of the transmitting laser. The use of the 1550-1560 nm band for the amplification of the analogue signal enables distortion of the analogue signal, due to this frequency variation of the transmitter laser, to be reduced thanks to the fact that the gain slope of the EDFA element is gentler in this part of the spectrum.
In one embodiment of the invention, the optical branch handling the digital signals has first and second amplifying portions (advantageously, embodied as respective EDFA coils) and a gain flattening filter arranged between these two amplifying portions. With this arrangement, the position of the gain flattening filter can be selected such that the best compromise can be reached between low noise and optimal output power for both upstream and downstream channels.
It is also preferred that two lasers should be used to drive the amplifying means in the parallel analogue and digital paths. In this way, the gain of each path can be controlled independently of the other. It is particularly preferred that each of the lasers should drive a respective amplifying means in the digital path whereas only one of the lasers drives the amplifying means in the analogue path.
Preferably the parallel digital and analogue paths are joined together at their ends by wavelength division multiplexer/demultiplexer devices. Not only do such devices enable the digital and analogue signals to be separated and recombined as desired, but also they reduce multiple path interference induced cross-talk and improve the noise figures of both analogue and digital sections by filtering the amplified spontaneous emission (ASE). It is also desirable that an optical isolator be included in the analogue branch, when the latter handles unidirectional signals.
In order to promote compactness of the overall apparatus, it can be useful to integrate with the optical amplifier apparatus of the present invention, the first signal splitter of the distributive network.